While various types of liquids are known as coolants used for cooling of automobile engines or the like, water is particularly preferable because of its cooling performance as an engine coolant, which is the highest among various types of liquids. However, fresh water freezes at 0° C., and its volume increases. Accordingly, fresh water may cause damage to engines or radiators. Under such circumstances, a coolant composition that was prepared by diluting a glycol base such as ethylene glycol with water so as to achieve a non-freezing temperature, optionally comprising various additives for protection against deterioration of metal, rubber, resin, or other materials used for engines or radiators has been used. In contrast, a coolant composition comprising water and various additives is occasionally used in areas in which there is no risk of freezing.
When a glycol such as ethylene glycol is used, disadvantageously, the viscosity of a coolant composition is significantly increased, particularly at low temperatures. In conventional techniques intended for improvement in viscosity characteristics, in general, viscosity reduction had been attempted in order to enhance fluidity at low temperatures (Patent Documents 1 to 3).
When viscosity reduction is attempted, however, the thickness of the boundary between the coolant and the bore wall is reduced, and convection is likely to take place. Thus, a coolant can easily conduct heat away from the bore wall, and cooling loss is increased as a consequence, which raises the new problem of decreasing energy efficiency. When a glycol such as ethylene glycol is concentrated to increase the viscosity of the coolant at low temperatures with the intention of lowering the heat release rate and reducing the cooling loss, however, cooling performance at high temperatures becomes insufficient, which disadvantageously causes overheating.
For example, Patent Documents 4 to 6 each describe a technique for improving viscosity characteristics of a lubricating oil by incorporating an agent for improving the viscosity index. However, such agent is incorporated with the intention of suppressing the decrease in viscosity at high temperatures and maintaining fluidity at low temperatures. With the use of a coolant comprising such agent, accordingly, it is not possible to reduce cooling loss at low temperatures and simultaneously maintain cooling performance at high temperatures.